Adjusting Work Function Of ITO By Modification Of Aliphatic Amine: Toward High Performance Polymer Solar Cells

Developing solution-processing interlayer is of highly important issues for high performance and low-cost polymer solar cells(PSCs). Up to date, many materials have been used as interlayers, such as inorganic materials of TiOx, ZnO and MoO3 etc; metal chelates of TIPD and ZrAcac etc; and organic compounds of PEI, PEIE, PCBDAN, PFN, PEDOT:PSS etc. Aliphatic amine though is an insulating material, has been successfully used as buffer layers. The insight study of mechanisms behind this interesting phenomenon would serve as a guideline for exploring new materials for this same purpose. In addition, to avoid the solvent orthogonality, the covalent bonding of the buffer layers to the electrode surface would greatly improve the operability, so that increase the reproducibility of the devices.1. In the first part, we presented state of the art and research background of this thesis.2. In the second part, the effect of PEI cathode interlayer on the work function and the interface resistance of ITO electrode in the inverted PSCs based on PBDTTT-C-T:PC70BM were introduced. It is found that a very thin layer of PEI(≤ 5.5 nm), either linear PEI(l-PEI) or branched PEI(b-PEI) with different molecular weights, is enough to lower the work function of the ITO electrode and to enhance the photovoltaic performance of devices. The champion power conversion efficiency(PCE) of devices with PEI cathode interlayers is 7.84%, more than doubled of that without these interlayers. However, a thicker PEI interlayer(≥ 10 nm) results in abrupt decrease of PCEs due to the increase of the resistance. Interestingly, for thicker interlayers, the l-PEI shows high photovoltaic performance than that of b-PEI, which can also be explained by their difference in resistances. This work supplies an insight into the function of PEI cathode interlayer on improving work function and resistance of ITO electrode in the inverted PSCs, and provides some instructions on the future design of interlayer materials in PSCs.3. In the third part, we demonstrated a new series of anode buffer layers, such as Dopamine, Dopa, Dobd, Npph. Two solvents have been used in the modification of ITO: one is Tris buffer(pH = 8.5), and the other is hydrochloric acid(pH = 1.5). It is found that the former caused a little change of work function, while the latter elevated the work function of ITO to 5.1 eV. The increased work function matches well with the HOMO energy of PBDTTT-C-T, thus makes it a good anode buffer layer. The PSCs with device configuration of ITO/Cathode interlayer/PBDTTT-C-T:PC70BM/LiF/Al was fabricated and characterized. Without this anode interlayer, the PCE of device is about 3.24%. With insertion of the these buffer layers, PCEs of devices are all higher than 6.3%. The control, PEDOT:PSS as anode interlayer, showed a PCE of 7.40%. Although the conversion efficiency is slightly lower that the classical PEDOT:PSS buffer layer, due to the great advantage of low cost of the catechol series, they can be a good candidate of the solution-processing anode interlayers.